I wouldn't worry much about the water... I know I already PMed you most of the info below, but will reiterate here for anyone else that might do a search and come across this info
The Mississippi Map Turtle I posted a picture of on the first page is housed at work in a 300 gallon rubbermade stock tank. He lives with several red-ear sliders, yellow belly sliders, malayan box turtle, asian pond turtle, common snapper, and a 4' American Aligator. All of the turtles are illegal in the state of Oregon, but we have one of three current permits in the state that allow us to keep them, so we end up them...
In the tub we keep water temperatures around 75F. Granite and Marble are stacked to create a basking area over which there is a 160W UVB light and a 150W CHE. The CHE is on 24/7, and the UVB is on only 12 hours a day.
The lil map turtle LOVES to eat. He routinely gets Mazori Aquatic Turtle pellets (we have a 25 cent gumball machine full of it to allow customers to feet the turtles and gator). He also eats the following straight from tongs - beef liver, chicken liver, chicken gizzards, pinky mouse parts, trout, crickets, mealworms, earthworms, and whatever else we pick up for feedings.
Even though he's the smallest guy in the pond, he definately isn't at the bottom of the totem. In fact, I've seen him scale over 3 other piled turtles to bask. I've seen him take shots at the gator, and he's plenty fast to get out of the way if he feels the need. He's my favorite in the pond, and the seconded in the store only by a MataMata...